The present invention is directed to a method for monitoring existing telephone connections, particularly permanently switched telephone connections.
In the prior art monitoring of telephone connections for error-free through-connection has been undertaken in the form of a call-associated cross office check (COC). Check bit patterns are transmitted after the implemented call set up from the line interface and line location of the calling subscriber to the line interface and line location of the called subscriber. The check bit patterns are mirrored back from the latter to the line location of the calling subscriber before a through-connection to the other subscriber is established. A through-connection or continuity fault is present insofar as the received check bit patterns do not correspond to the transmitted check bit patterns. A limiting of the monitoring of telephone connections to the described monitoring undertaken before the actual through-connection is not satisfactory, particularly for permanently through-connected connections, what are referred to as nailed-up connections (NUC connections). An interruption of such connections and a subsequent renewed setup merely for the purpose of implementing the described check of the path through-connection does not come into consideration.
In the prior art an error search given such nailed-up connections could only be undertaken in response to corresponding subscriber complaints, resulting in a relatively complicated fault elimination.
European Patent Application 87 108 196.4 discloses, first, that the information transmitted from the redundant switching network parts to the line side be immediately compared to one another bit-by-bit for the purpose of monitoring nailed-up connections and, second, that the information to be forwarded be provided with a parity bit and be subjected to a parity check after traversing all units of the periphery. Alternatively, the information in the line interface of the periphery coming from connected subscriber lines or transmission lines be provided with a parity bit and is likewise subjected to a parity check after traversing the peripheral units of the switching center. Such a testing strategy requires a relatively high outlay and is not especially suitable particularly given an organization of the switching center periphery into assemblies having a plurality of input and output terminals.